Hydrocarbon synthesis catalysts, also known as Fischer-Tropsch catalysts, have been studied widely and by various researchers in the past sixty years. Preferred processes now usually employ cobalt or ruthenium, cobalt and ruthenium, or promoted cobalt catalysts. The catalysts are supported on a variety of supports, but usually they are supported on inorganic oxides, such as alumina, silica, titania, silicaalumina, and the like.
Promoters can be used to enhance the activity or stability of cobalt or ruthenium catalysts and these promoters may vary. For example, rhenium has been used to promote cobalt catalysts supported on either titania or alumina, see U.S. Pat. No. 4,568,663 and U.S. Pat. No. 4,801,573 respectively. Supported ruthenium catalysts are also quite useful for hydrocarbon synthesis (see U.S. Pat. Nos. 4,477,595; 4,171,320; and 4,042,614). Also, ruthenium and zirconium have been used to promote cobalt supported on silica (see U.S. Pat. Nos.4,088,671, 4,599,481, and 4,681,867). Two stage hydrocarbon synthesis was disclosed in U.S. Pat. No. 4,443,561 relating to hydrogen:carbon monoxide ratios, but making no differentiation based on the pressure in each reaction stage. Suffice to state that a variety of cobalt containing catalysts have been disclosed for hydrocarbon synthesis processes operating over a relatively wide pressure range. However, to achieve practical results from a hydrocarbon synthesis process, in the sense of converting carbon monoxide and obtaining the maximum availability of desired products, usually C.sub.5 + products, these processes are conducted in at least two serially connected stages. That is, with the exception of liquid removal, the second stage operates on the products of the first stage at essentially the outlet pressure of the first stage.
Other two stage hydrocarbon synthesis processes have been reported in the literature. For example, EPA159759, published Oct. 30, 1985 employs a first stage cobalt catalyst and a second stage catalyst having water gas shift activity, while EPA1121.951 published June 27, 1984 employs a second stage catalyst with activity for converting olefinic or oxygenated products to isomeric hydrocarbons. Also, several e.g., U.S. Pat. Nos. 4,547,609, 4,279,830, and 4,159,995 use an iron based first stage catalyst for hydrocarbon synthesis and a second stage catalyst having activity for aromatization. Also, U.S. Pat. No. 4,624,968 employs an iron based first stage catalyst for producing olefins and a second stage catalyst for converting olefins to paraffins with additional CO and hydrogen. All of these systems are based on dual function catalyst systems, that is, where the first stage catalyst is active for a specific chemical reaction and the second stage catalyst is active for a different chemical reaction. However, none of these systems involve a two stage process in which catalysts of essentially equivalent functionality are tailored to the specific operating conditions of each stage.
Hydrocarbon synthesis processes are known to be plagued with several problems. Of these problems, obtaining high conversion and dissipating heat are among the foremost. Since hydrocarbon synthesis is an exothermic reaction, heat must be removed from the reactor to avoid hot spots, catalyst deactivation, and loss of selectivity at higher temperatures. There is usually a preferred temperature range for operating the process which leads to the optimum selectivity to desired higher hydrocarbon products. Lack of efficient heat removal can lead to much higher temperatures in the reactor which, while increasing carbon monoxide conversion, severely debits the selectivity to preferred higher hydrocarbons. At the same time, increasing conversion generates more heat and, thus, a greater burden on heat exchange facilities. Thus, the goals of high conversion and efficient heat transfer tend to oppose each other. To alleviate the problem, lower conversion in a first stage can be accommodated, thereby, reducing the heat load in the first stage. However, this reduced conversion must be made up in the second stage.
Now, increasing pressure for a given reaction and catalyst system generally increases carbon monoxide conversion in hydrocarbon synthesis. However, there is a pressure drop across the first stage reactor and achieving adequate conversion in a second stage can require intermediate compression of the unreacted synthesis gas, an expensive step.
This invention takes advantage of the finding that cobalt on alumina catalysts, specifically cobaltrhenium on alumina is a more active hydrocarbon synthesis catalyst at low pressures than other commonly used catalysts.